1. Field of the Invention
The present invention relates to a furnace for growing multiple crystals of silicon, more particularly, to a crystal-growing furnace with a convectional cooling structure.
2. Description of Related Art
Referring to FIG. 1, a schematic view illustrating a conventional crystal-growing furnace, a heating room 91 is provided inside the crystal-growing furnace, where an insulating cover 911, a heater 912, a crucible 913, and a supporting table 914 is arranged in the heating room 91.
As shown in FIG. 1, silicon slurry is heated, by the heater 912, to a molten state, and when in a crystal-growing process, the insulating cover 911 is lifted so that cooling gaseous stream can be introduced into the heating room 91 from under the insulating cover 911. However, the supporting table 914 has a great bulk, and so a relatively great heat capacity, making it difficult for the underneath of the crucible 913, loaded with the silicon slurry, to be cooled despite the cooling gaseous stream. As a result, the silicon slurry at the bottom of the crucible 913 cannot have a desirable crystal-growing temperature, though the silicon slurry at the upper and side parts, in particular corners, of the crucible has been cooled and thus solidified already.
When the silicon slurry turns from a liquid phase into a solid phase, the volume thereof expands about 9.5%. In the conventional crystal-growing process of silicon slurry, crystal growth starts from the upper part of the crucible 913, then to the side part and eventually to the central part of the crucible 913. That is to say, the silicon slurry at the upper part and the peripheral part of the crucible will be solidified first, leaving an expansion pressure, produced from the later-solidified silicon slurry at the central part of the crucible 913, cannot be released upward and out of the upper part of the crucible 913, and as such, an internal stress is formed. Such internal stress will now and then cause fracture around the crystal ingots, particularly more serious at the corners of the crystal ingots. Even if the fracture is not obvious, the crystal ingots or chips might be fractured when a later cutting process is performed. As a result, the production of crystal ingots fails to have a desirable quality.
Moreover, during the crystal-growing process, the insulating cover 911 is lifted up, making inner wall of the insulating cover 911 rub against an upper board 910, such that bits of graphite will be peeled off and thus contaminate the crystal ingots of silicon. When annealing, the insulating cover 911 is lowered down so as to close the heating room 91, and is lifted up again for cooling purpose. Such moving up and down of the insulating cover 911 makes bits of graphite fall into the silicon slurry which thus contains more carbon, and as a result, quality of the crystal ingots of silicon is downgraded. Further, since the inner wall of the insulating cover 911 rubs against the upper board 910 for quite a long time, a gap between the inner wall of the insulating cover 911 and the upper board 910 becomes greater, and that the situation of heat loss gets seriously day by day.
Still further, in order to shorten the duration of cooling, six side partitions of the heating room 91 are constructed by a single heat insulating layer 92 made of graphite, without any additional insulating layer to keep warm the heating room 91. Nevertheless, such a heat-insulating measure with a thin layer cannot gather heat effectively. In particular, after underneath of the insulating cover 911 is opened, it is necessary to lower temperature for the bottom of the crucible 913 so as to grow crystal uniformly from the crystal slurry. In the meantime, a greater electric power is required for maintaining high temperature at the upper part of the heating room 91. These make the crystal-growing process consume more electricity and time.